1. Field of the Invention
The present invention relates to a substrate for electronic devices, a manufacturing method therefor, and an electronic device. More specifically, the substrate for electronic devices according to the present invention is preferably used for mounting thereon a ferroelectric element functioning as a capacitor, a piezoelectric element functioning as a cantilever, or the like.
2. Description of the Related Art
Recently, the development of ferroelectric memories, being non-volatile memories using a ferroelectric, is proceeding rapidly. Ferroelectric memories are divided into; a capacitor type using a ferroelectric as a capacitor, formed in a 1T (transistor)/1C (capacitor) structure, and an MFSFET (Metal Ferroelectric Semiconductor Field Effect Transistor) type using a ferroelectric as a gate insulating film for a field-effect transistor instead of SiO2. The MFSFET type is more advantageous than the capacitor type in view of high integration and non-destructive reading, but it has not yet been realized due to difficulty in manufacturing related to the structure. Hence, development and commercialization of the capacitor type is ahead at present.
Representative ferroelectric materials adopted in the capacitor type ferroelectric memory include PbZr1−xTixO3 (abbreviated as PZT) and SrBi2Ta2O9 (abbreviated as SBT). Of these, the PZT having a composition near a rhombohedral and tetragonal phase boundary (Morphotropic Phase Boundary; abbreviated as MPB) has excellent remanence and anti-field characteristic, and is put to practical use.
The capacitor type ferroelectric memory has a structure in which the ferroelectric material PZT, is placed between a lower electrode and an upper electrode. Pt has heretofore been used as a material forming the lower electrode. Having a face-centered cubic lattice structure, which is the closest packing structure, Pt has strong self-orientation, and is oriented in a cubic (111) direction even on a thin film having an amorphous structure, such as SiO2 and is hence used preferably. However, since Pt has strong orientation, when columnar crystals grow, there are problems in that Pb or the like is likely to diffuse in the foundation along the grain boundary, and the bond between Pt and SiO2 becomes poor. As one example of measures against the problems, Ti is used for improving the bond between Pt and SiO2, and TiN is used for preventing Pb from diffusing, in many cases. However, when Ti or TiN is used, the lower electrode has a complicated electrode structure, causing oxidation of Ti and diffusion of Ti into Pt, accompanied with deterioration in crystallinity of PZT. As a result, deterioration in polarization-electric field (P-E) characteristic, leak current characteristic and fatigue characteristic (tolerance to repeated write) may be caused.
In order to avoid various problems when Pt is used as the lower electrode, research for using conductive oxides, as represented by RuOx and IrO2, for the lower electrode material has been conducted. Among these materials, SrRuO3 having a perovskite structure has the same crystalline structure as that of PZT, and hence has excellent bondability on the interface and excellent characteristic as a diffusion barrier layer for Pb, and can easily realize the epitaxial growth of PZT. Therefore research into ferroelectric capacitors using SrRuO3 for the lower electrode is being actively conducted.
However, in the case of the ferroelectric capacitor having a construction where an oxide having a perovskite structure, such as SrRuO3, is used for the lower electrode, and PZT is provided thereon as a ferroelectric, there are problems as described below.
It is important for PZT to have a composition with more Ti than MPB which has a composition of Zr:Ti=0.52:0.48, for example, a composition of Zr:Ti=0.3:0.7, from a standpoint of an increase in remanence Pr and a decrease in the anti-electric field Ec. However, PZT in this composition range exhibits a tetragonal, and the polarization direction thereof is parallel with the C-axis. As a result, in a ferroelectric capacitor having a structure in which a lower electrode, a ferroelectric and an upper electrode are laminated in this order on a substrate, it is necessary to orient the SrRuO3 electrode itself, being the lower electrode, pseudo-cubically (100), in order to allow the PZT forming the ferroelectric layer to be a (001) oriented film.
However, when an electrode consisting of SrRuO3, being a perovskite type oxide, is directly deposited on an Si substrate, an SiO2 layer is formed on the interface therebetween. Hence, it is difficult to grow SrRuO3 epitaxially. Therefore, there has been studied a method in which some kind of buffer layer is grown epitaxially on the Si substrate beforehand, and an SrRuO3 electrode is grown epitaxially on the buffer layer (for example, see Patent Document 1).
Here the buffer layer epitaxially grown on the Si substrate includes oxides having a fluorite structure, such as yttria stabilized zirconia (abbreviation: YSZ, Zr1−xYxO2−0.5x) and CeO2. These materials have been reported, for example, in non-patent document 1 for YSZ, and in non-patent document 2 for CeO2/YSZ.
(Patent Document 1)
Japanese Unexamined Patent Application, First Publication No. 2001-122698
(Non-Patent Document 1)
Appl. Phys. Lett., vol. 57 (1990) 1137
(Non-Patent Document 2)
Appl. Phys. Lett., vol. 64 (1994) 1573
The present inventors have conducted research and development for a case where a YSZ buffer layer is used, and the SrRuO3 electrode is grown epitaxially thereon, and as a result, it has been found that this structure has two problems described below.
The first problem is that in order to grow a buffer layer such as YSZ and CeO2 on the Si substrate epitaxially, predetermined surface treatment is necessary with respect to a film-forming surface of the Si substrate, before deposition of the buffer layer thereon. Conventionally, as this surface treatment method, two methods are well known, that is, a method of forming a reconstructed surface, and a method of forming a hydrogen-terminated surface. For example, a technique in which a surface of the Si substrate is changed to a hydrogen-terminated surface, by subjecting the surface of the Si substrate to a treatment by hydrofluoric acid is disclosed in non-patent document 3. Here, the reconstructed surface represents a surface in which the periodic structure of the surface has been changed to bulk, by performing heat treatment in a high temperature and high vacuum atmosphere, so as to allow excessive covalent bonds (dangling bonds) in Si atoms forming the surface layer of the Si substrate to bond with each other. On the other hand, the hydrogen-terminated surface stands for a surface having a structure in which after an SiO natural oxide film forming the surface layer of the Si substrate is etched by washing with hydrofluoric acid, the dangling bonds on the surface are terminated with hydrogen in ammonium fluoride solution.
(Non-Patent Document 3)
Appl. Phys. Lett., vol. 57 (1990) 1137
The second problem is that the orientation direction of SrRuO3 formed on the buffer layer changes to the (110) orientation (pseudo-cubic). It is well-known that when SrRuO3 having a simple perovskite structure (in orthorhombic, a=0.5567 nm, b=0.5530 nm, c=0.7845 nm, and in pseudo-cubic, a=0.3923 nm, 21/2a=0.5548 nm) is provided on the (100) surface of YSZ (a=0.514 nm) or CeO2 (a=0.541 nm) having a fluorite structure, epitaxial growth does not occur in the (100) orientation, but is (110) oriented (pseudo-cubically) (for example, see non-patent document 4)
(Non-Patent Document 4)
Appl. Phys. Lett., vol. 67 (1995) 1387
In other words, in order to epitaxially grow the buffer layer such as YSZ or CeO2 on the Si substrate, predetermined surface treatment has heretofore been required with respect to the film-forming surface of the Si substrate, before deposition of the buffer layer thereon, thereby causing complication of the manufacturing process and an increase in the manufacturing cost. Even if such predetermined surface treatment is conducted, the epitaxial growth of the buffer layer formed thereon cannot be realized in the desired (100) orientation, but only the epitaxial growth in the (110) orientation can be realized.